1. Field of the Invention
The present invention relates to a composition for a solar cell sealing film consisting chiefly of ethylene-polar monomer copolymer which is suitably utilized in preparation of a solar cell.
2. Description of the Related Art
A solar cell (solar cell module) has been widely employed as a device directly converting solar energy into electric energy from the view-points of effective use of natural resources and ecology, and is further developed.
As shown in FIG. 1, a solar cell has a structure that plural photovoltaic elements 14 (e.g., photovoltaic elements made of silicon) are sealed through a front side sealing film 13A and a backside sealing film 13B between a transparent front side protection material 11 and a backside protection material 12 (backside covering member).
In a conventional solar cell, a transparent substrate such as a glass plate is used as the transparent front side protection material such that light incident upon the solar cell is effectively taken into the photovoltaic elements of the solar cell as much as possible. In contrast, a plastic film such as polyethylene terephthalate (PET) or a plastic film having a silver-deposited layer thereon is used as backside protection material in order to prevent water from invading the inside of the solar cell.
As a front side sealing film or a backside sealing film, a film made of ethylene-polar monomer copolymer such as ethylene-vinyl acetate copolymer (EVA) or ethylene-ethylacrylate copolymer (EEA) is used. The ethylene-vinyl acetate copolymer is preferably used in a conventional solar cell because it has low cost and high transparency.
A solar cell (module) is generally prepared by extending an EVA composition comprising EVA and a crosslinker by the application of pressure and heat to form a sealing film, and thereafter superposing a transparent front side protection material, a front side sealing film, photovoltaic elements, a back side sealing film and a backside protection material in this order and curing or crosslinking EVA by heating under pressure at temperature of 135 to 180° C. to combine them.
The conventional solar cell sealing film is crosslinked by using a crosslinker in addition to ethylene-polar monomer copolymer to enhance crosslink density for the purpose of improvement of the strength or durability (Patent Document 1). As a method to activate the crosslinker, a thermal decomposition method, a redox decomposition method and an ion decomposition method are known. The thermal decomposition method is generally used.    Patent Document 1: Japanese Patent Number 3,473,605